嵌套泛型类型参数参数

Question

I have an abstract class which is defined as:

public abstract class BaseClass<T extends FirstClass, U extends BaseAnother<? extends SecondClass>> {

I don't wanna use question mark ( ? ) and use some generic variable as like T or U .

How can I do that?

Answer 1

It is public abstract class BaseClass<X extends SecondClass, T extends FirstClass, U extends BaseAnother<X>> { , no?

(I've added X extends SecondClass )

Think about how you will use this type variable. The way declare it when defining class is to path it in new BaseClass< /* type parameters - here */> { ... } .

Also note, according to JLS , type parameters are declared in class definition in one place:

NormalClassDeclaration: ClassModifiersopt class Identifier TypeParametersopt Superopt Interfacesopt ClassBody

See example there below from 8.1.2. Generic Classes and Type Parameters chapter:

interface ConvertibleTo<T> {
    T convert();
}
class ReprChange<T extends ConvertibleTo<S>,
                 S extends ConvertibleTo<T>> { 
    T t; 
    void set(S s) { t = s.convert();    } 
    S get()       { return t.convert(); } 
}